1. Field of the Invention
This invention relates to a device for driving a lens unit, such as a telephoto lens, having a tripod mount and which is driven by a drive source provided on the side of a camera body, so as to be focused on an object.
2. Background of the Invention
Heretofore, the construction of a telephoto lens with a tripod mount is such that the lens barrel is freely rotatable with respect to the tripod mount. Roughly stated, there are two types of construction available for such telephoto lenses. In one of the two types of constructions, the telephoto lens is turned at the tripod mount. In the other type of construction, only the mount part and the automatic aperture control mechanism are turned. The former type of construction is simple. However, in this type of construction, the total weight of the lens is supported only by the lens-turning part of the tripod mount. Also, in order to prevent blurring which may be caused when the shutter is operated, the tripod mount is, in general, spaced away from the center of gravity of the lens, with the result that a relatively large moment of force is required to turn the camera. Accordingly, when it is required to change the photographic composition by turning the camera, the camera must be turned with a relatively large force. Therefore, it is rather troublesome to operate the camera with the telephoto lens of this type. On the other hand, in the case of the latter type of construction, even if the lens is large both in diameter and in weight, there are only a few parts to be turned, and therefore the photographic composition can be readily changed.
An automatic focusing single-lens reflex camera in which the focusing lens in the lens barrel is driven by a drive source built in the camera body has been highlighted recently. What is obtained by applying this mechanism of the reflex camera to a telephoto lens having the latter type of construction is a lens barrel shown in FIG. 3. However, the lens barrel is disadvantageous in that, when it is required to turn the lens barrel to change the photographic composition, the lens barrel must be turned through a relatively large angle, and the rotation of the lens barrel causes the lens to be out of focus.
The construction of the telephoto lens barrel shown in FIG. 3 and its problems will be described in detail.
As shown in FIG. 3, a stationary cylinder 3 is secured to a tripod mount 1 with screws 2 which is mounted on a tripod (not shown). A rotatable cylinder 4 is rotatably supported within the stationary cylinder 3. A protrusion 5 is formed on the outer wall of the rotatable cylinder 4 and is held in contact with an annular guide 6 secured to the stationary cylinder 3 so that the rotatable cylinder 4 is rotatable with its axial movement inhibited. The tripod mount 1 has a locking member 7 for locking the rotatable cylinder 4 to the stationary cylinder 3.
The stationary cylinder 3 has a small-diameter part 8 which is engaged with the inner surface of an inner cylinder 10 of a drive ring 12. The drive ring 12 is made up of the inner cylinder 10, an outer cylinder 9 and a flange 11 through which the inner cylinder 10 is connected to the outer cylinder 9. The axial movement of the drive ring 12 is prevented by a cylinder 13 which is secured to the small-diameter part 8 of the stationary cylinder 3 with screws. The angle of rotation of the drive ring 12 is regulated by a stopper 14 which is secured to the cylinder 13 with screws.
A lens frame 16 supporting a movable focusing lens 15 is mounted within the inner cylinder 10 in such a manner that it is movable in the axial direction and rotatable around the axis.
In order to convert the rotation of the drive ring 12 into the axial movement of the lens frame 16 (and accordingly of the lens 15), a straight movement guide groove 17 is formed in the inner cylinder 10 of the drive ring 12 and a cam groove 18 is formed in the small-diameter part 8 of the stationary cylinder 3. A pin 19 embedded in the lens frame 16 is engaged with the straight movement guide groove 17 and the cam groove 18. Therefore, the lens 15 is moved by manually turning the outer cylinder 9 of the drive ring 12.
A drive shaft 24 is rotatably supported in a close fitting cylindrical hole in the rotatable cylinder 4 so that the lens 15 can be driven also by the motor built in the camera body. The joint 25 is of the type that, unless driven, it maintains the orientation of the drive shaft 24 with respect to the rotatable cylinder 4 even when the rotatable cylinder 4 is rotated. When the joint 25 is driven by the motor from the side of the camera body at any rotational position of the rotatable cylinder 4, it rotates the drive shaft 24 relative to the rotatable cylinder.
The drive shaft 24 has the joint 25 at the rear end. The joint 25 is located at a mount part 20 so as to be engaged with the camera body. The drive shaft 24 has a gear 23 at the front end. The gear 23 is engaged with an intermediate gear 22 which is rotatably fitted on a supporting ring 21 which is threadably engaged with the stationary cylinder 3. A cooperating shaft 26 rotatably supported by the stationary cylinder 3 has a gear 27 at one end. The gear 27 is engaged with the intermediate gear 22. Its shaft 26 has a gear 29 at the other end which is engaged with a gear 28 formed on the inner cylinder 10 of the drive ring 12. Therefore, the lens can be driven also by the power of the drive source built in the camera body.
However, because of the drive shaft 24 provided in the rotatable cylinder 4, the telephoto lens barrel shown in FIG. 3 suffers from the following difficulties.
As was described above, the joint 25 of the drive shaft 24 is connected to the joint (not shown) of the camera body. Therefore, when the rotatable cylinder 4 is rotated in order to change the composition, the drive ring 12 is also turned through the intermediate gear 22 and the cooperating shaft 26 so that the lens 15 is moved, that is, it is moved out of focus. If the rotatable cylinder 4 is turned with the rotation of the drive ring 12 prevented with the stopper 14a, the drive shaft 24 is turned because in this case the intermediate gear 22 is held fixed.
However, depending on the rotation of the drive shaft, considerably large torque is required to turn the joint of the camera body, because, as was described above, the joint 25 of the drive shaft 24 is coupled to the joint of the camera body, which is generally coupled to the motor and reduction gear train system which is so arranged as to accelerate the motor as viewed from the lens. Therefore, when the rotation of the drive ring 12 is stopped by the stopper 14a, immediately it becomes difficult to turn the rotatable cylinder 4.